In these years, need for electrical products has been greatly increased. Auto shutdown functions of the electrical products are very important for reducing the power consumption and for increasing the lives of electrical products. An auto shutdown function can be performed by a power sleep-mode control. For example, the power sleep-mode of a monitor can be set in a personal computer. In this case, the monitor can be switched to the power sleep-mode under the condition that no input signals to the monitor are generated from a keyboard or a mouse during a predetermined period of time set by the user. Therefore, the power consumption can be reduced, and the life of the monitor can be prolonged. Similarly, a hard disk in a computer, a printer, a power supply, or other electrical products can be switched to their power sleep-modes.
A conventional DC brushless motor is usually used for driving a cooling fan in a computer or a workstation. The cooling fan operates when the computer or workstation is switched on till the time when the computer or workstation is switched off or when the fan fails to operate. In general, an operation period of a computer or workstation is very long. When any part or electrical element of a computer is switched to its power sleep-mode, the corresponding cooling fan can reduce its rotating speed or even stop in order to reduce power consumption and prolong its life.
The power sleep-mode has not been utilized in conventional cooling fans for saving power consumption and prolonging their lives.
Conventionally, a pulse width modulation (PWM) method is widely used for controlling the rotating speed of a cooling fan. According to the method, the rotating speed is controlled by modulating the amount of voltage or current inputted to a DC motor of the cooling fan. However, a circuit for carrying out this method is complicated and consumes a lot of electrical power.